Wideband communications devices typically operate with a wideband complex signal, and can interfere with narrowband communications devices. Interference with narrowband communications devices may cause a loss of critical communication links. For wideband and narrowband communication devices to coexist, the wideband complex signal being transmitted may be filtered to generate notches overlapping the operating frequencies of the narrowband communication devices.
An example wideband communications device is a satellite communications system known as MUOS (Mobile User Objective System). The MUOS satellite communications system operates in the UHF band. The MUOS system is designed to reduce the impact on nearby narrowband communications devices by using an adaptive notch-on-transmit filter.
There are several approaches for providing an adaptive notch-on-transmit filter for notching the MUOS waveform. Each approach uses a different filter bank structure that affects processor loading and memory requirements. For example, a paper titled “Filterbanks For Adaptive Transmit Filtering” by Chad Spooner discloses notch-on-transmit algorithms that focus on a DFT filterbank and on a modified DFT (mDFT) filterbank.
The DFT filterbank is applied to the waveform in a sliding block manner. In the mDFT, the input signal is simultaneously applied to each of the sub-band branches of the filterbank. Analysis filters are immediately applied to the input data. Each analysis filter is a frequency-shifted version of a real-valued low pass filter. The outputs of the analysis filters are decimated, and the real and imagery parts of the result are alternately taken over time. The parallel operations result in real and imagery components for each input block of complex numbers, just as in the DFT filterbank. The difference between the mDFT and the DFT filterbanks is that the exact sub-band filtering characteristics are under a designer's direct control in the mDFT filterbank. Even in view of the DFT filterbank and the modified DFT (mDFT) filterbank, processor loading and memory requirements are not significantly reduced.
A paper titled “MUOS Spectrum Notching Effect On Handheld Terminal Uplink Performance” by Kumm et al. discusses the effect of the peak-to-average power ratio (PAPR) of the spectrally adaptive waveform in terms of performance. The PAPR increases with an increasing notching bandwidth in the spectrally adaptive waveform as compared to an un-notched waveform. With spectrally adaptive notching, the dynamic range of the communications device transmitting the waveform is “squeezed” by the need to preserve PAPR while meeting a maximum power limit. The paper concludes that there is no straightforward way to reduce PAPR into the power amplifier of the communications device transmitting the waveform so as to boost output power of the required notch depth and out-of-band requirements. In addition, the paper fails to address reducing processor loading and memory requirements when generating the notching in the spectrally adaptive waveform.